Fewer calls to `GET_EC()` and `GET_THREAD()` (#15506)

The changes are to `io.c` and `thread.c`.
I changed the API of 2 exported thread functions from `internal/thread.h` that
didn't look like they had any use in C extensions:

* rb_thread_wait_for_single_fd
* rb_thread_io_wait

I didn't change the following exported internal function because it's
used in C extensions:

* rb_thread_fd_select

I added a comment to note that this function, although internal, is used
in C extensions.

0 files changed, 0 insertions, 0 deletions